Zebras are famous for their contrasting black and white stripes – but until recently, no one really knew why they wore their unusual striped pattern. This is an issue that has been debated 150 years ago by great Victorian biologists such as Charles Darwin and Alfred Russel Wallace.
Since then, many ideas have been put on the table, but it is only in recent years that serious attempts have been made to test them. These ideas fall into four main categories: zebras are striped to avoid capture by predators, zebras are striped for social reasons, zebras are striped to stay cool or they have stripes to prevent biting flies .
Only the last one resists the examination. And our latest research helps to better understand why.
What is the advantage of zebra stripes?
Can scratching help zebras not become a predator meal? There are a lot of problems with this idea. Field experiments show that zebras stand out to the human eye when they are between trees or in grasslands, even when the lighting is poor – they seem far from being camouflaged. And when they flee danger, zebras do not behave in a way that maximizes any confusion that might be caused by striping, making hypothetical ideas about dazzling predators intangible.
Worse still for this idea, the sight of lions and spotted hyenas is much weaker than ours; these predators can only solve the lines when the zebras are very close, at a distance when they can probably hear or feel the prey anyway. It is therefore unlikely that the stripes are of great use in the anti-predator defense.
The most damaging zoo is the prey of lions – study after study across Africa, lions kill them more than one might think because of their numerical abundance. Stripes can not be a very effective anti-predator defense against this important carnivore. This is for the hypothesis of avoidant predators.
What about the idea that bands help zebras to interact with members of their own kind? Each zebra has a unique striped pattern. Could it be useful in individual recognition? This possibility seems highly improbable given that domestic horses of uniform colors can recognize other individuals at sight and sound. Striped members of the horse family do not groom themselves – a form of social bond – more than the un-scratched equine species. In addition, members of the group do not flee the very rare and bandless individual zebras that breed successfully.
What about some kind of defense against the hot African sun? Since black stripes could be expected to absorb radiation and white stripes reflect them, an idea has been proposed: they establish convection currents along the back of the animal, cooling it so.
Again, this seems unlikely: detailed experiments in which large water drums had been draped in skin of uniform color or striped, or painted with scratches or not, showed no difference in internal temperature of the water . In addition, thermographic measurements of wild zebras, impala, buffaloes and giraffes show that zebras are not colder than those other species with which they live.
The last idea about striping seems absurd at first sight – scratches prevent stinging insects from feeding on blood – but they have a lot of support.
Early experiments in the 1980s showed that tsetse flies and horse flies avoid landing on striped surfaces and have been confirmed more recently.
But what is most convincing is the data of the geographical area of the seven species of live equidae. Some of these species are striped (zebras), others not (Asian donkeys) and some are partially striped (wild ass of Africa). Within the species and their subspecies, the intensity of the streak is closely related to the discomfort caused by biting flies in Africa and Asia. That is, wild equidae native to areas where the agony of horse flies is prolonged during the year are those that are most likely to present marked streaks.
We believe that the reason that equids must be scratched in Africa is that African biting flies are carriers of diseases such as trypanosomiasis, African horse sickness and equine influenza, which can be fatal to equines. And zebras are particularly likely to probe by biting the mouthparts, because of their short coat. Having a fur pattern that prevented flies and the deadly diseases they carried would be a significant benefit, meaning that the stripes would be passed on to future generations.
Test the idea that stripes and flies do not mix
But how do stripes exert their influence on biting flies? We have undertaken to examine this question in a livery of Somerset, in the United Kingdom, where horse flies attach themselves in summer.
We had the chance to work with Terri Hill, the owner of the livery. We could get very close to his horses and tame plains zebras, allowing us to watch the flies land or fly over the equines. We also filmed the behavior of flies around the animals and put coats of different colors on the horses.
It is important to remember that the vision of flies is much worse than that of people. We found that zebras and horses received a similar number of sandfly approaches, probably attracted by their scent – but that zebras had far fewer landings. Around the horses, flies flutter, spiral and turn around before landing again and again. On the other hand, around the zebras, the flies flew over or made a single fast landing before leaving.
Frame-by-frame analysis of our videos showed that flies slowed slowly as they approached brown or black horses before making a controlled landing. But they did not decelerate when zebras approached. Instead, they were flying straight ahead or, literally, hitting the animal and bouncing.
When we place black coats or white coats or striped coats on the same horse to control any difference in behavior or smell of an animal, flies do not arise on scratches either . But there was no difference between the landing rates on the bare head of the horse, which shows that the rays exert their effect closely, but do not interfere with remote fly approaches.
And that showed us that striped horse coats, currently sold by two companies, really work.
So now that we know that scratches affect the very close horse flies, not remotely, what really happens to a few inches from the host? One idea is that tapes create an optical illusion that disrupts the expected pattern of movement of the fly at the approach of the zebra, preventing it from landing properly. Another idea is that flies do not see the zebra as a solid entity but as a series of thin, black objects. It is only when they are very close that they realize that they are going to hit a solid body and that they are rather moving away. We are studying these possibilities now.
Thus, our basic research on fly behavior does not only tell us why zebras are so beautifully striped, but it does have real implications for the horse apparel sector, with the potential to make horse riding and maintenance of the horse less painful for the horse and the rider.
Tim Caro, Professor of Ecology of Wildlife, Fish and Conservation, University of California Davis and Martin How, University Researcher in Biological Sciences at the University of Bristol
This article is republished from The Conversation under a Creative Commons license. Read the original article.